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Timing Mech vs Vac advance Good Read

Questions, answers, tips and tech on drivetrain issues.

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Post by BIG BLOCK GP » Sun May 23, 2004 9:46 am

thanks for all the info

my question is I have a BBC with a 254-260 660-666 solid roller cam that will make barly any vacum so is it worth me running vacum advance or not?

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Re: Timing Mech vs Vac advance Good Read

Post by Norm Peterson » Thu May 27, 2004 6:55 am

Little Red Wagon wrote:This is a reprint from another board author unknown but wanted to share this with the group as there has been some recent discussions on this.
. . .
For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.
Thanks for reposting this article, Fred.

I saw later on in this thread mention of an ECU, and I'll say right now that you absolutely need to understand all of that stuff if you ever get into tuning EFI ignition maps. But when you do understand it, EFI ignition tuning is pretty simple and at the same time gives you far greater control over the shapes of the "curves".

I wonder if the member who still had pinging issues regardless of which vacuum can he was using was encountering partial blockage in the vacuum line or if the canister itself wouldn't let the air back into it at enough of a flow rate to kill the vacuum advance fast enough? I'd look for debris in the lines, damaged fitting(s), perhaps a hose being collapsed shut or even being too small, and things like the hose connection at the can being too small for his specific combination or the vacuum can return mechanisms too soft for the can-related problems.

IOW, anything that could restrict or limit the airflow back into the canister needs to be investigated. Apparently his combination wants the vacuum advance "all gone, right now", and something (or several things) is not letting that happen.

1979 Malibu Classic (RIP)
2010 Subaru LGT (wife's DD, 6-speed manual)
2008 Mustang GT (Slightly unstock, 5-speed manual)
1995 Mazda 626 (bad winter weather driver and spare car, 5-speed manual, not stock either)
2001 Maxima SE (spare car, 5-speed manual, a little bit not stock, too)

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Post by Little Red Wagon » Thu May 27, 2004 7:45 am

Norm tuning is still hard for this old man but last year I finally switched to a crank trigger for my MSD setup. It is easier to control the spark for me with it than my other setup and more precise than the other was.


Post by Hilljack » Tue Jul 13, 2004 12:29 pm

I too Have a cam I don't think is going to make much Vacuum. What would be the minimum amount required to operate the lowest start HG. canister?

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I have neither

Post by Excelerater » Tue Jul 13, 2004 12:42 pm

My distrub is locked at 38 degrees

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i have a question regarding this as well

Post by SEVNT 9 » Thu Sep 16, 2004 6:02 pm

i have a 355 with milled vortec heads 10.5:1 crane cam .454 .480 216/228 @ .050 i set timing durring break in to 30 total at 3000 with vac. adv. unplugged (i was told to go to 32 but i have also heard vortecs perfer 28-30 i was fearing spark knock durring break in with all the extra heat so i played it safe ) i havent changed the position yet my readings on the timming light jumped all the way to 60 (balancer at 0 @ 3000)with vac plugged in. i have the line ran to the port that the instructions say to place timmed spark source is this ported or manifold vac. and should i go to 32 deg. with this combo.............. is more info needed?
I use to be "bu min v8"

'79 malibu classic, factory bucket/floorshift car: 355, vortec heads, Holley 670, Edelbrock RPM airgap, Comp extreme energy 268, Dynomax ceramic coated headers, 2.5" mandrel bent exhaust from Jegs, T56, 8.5 GN rear, 4.10 richmond gear, moser axles Monte ss booster & master.

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Post by DragginWagon » Tue Oct 05, 2004 2:51 am

my question is I have a BBC with a 254-260 660-666 solid roller cam that will make barly any vacum so is it worth me running vacum advance or not?

Personally with your combo, and I'm "assuming" it's not regularly street driven, more of a weekend warrior, then I'd lock out the dist.

I've been running the wagon like that for 2 years with NO problems and great driveability.

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Post by Dom87SS » Mon Jan 10, 2005 12:13 pm

the only question i have is about the weights and springs. is there somewhere or way to tell what the weight of the weight you are using is. and which you should, or would be better to use for a certain application. and spring rates, i know there are several different rates being soft, medium, and stiff. but how do you know whcih is for you. as someone mentioned they were using 2 different types of springs. i have a stock HEI housing, shaft, and cam gear. but all new pertronix everything else except the bushing and wires which are both MSD. i had gotten a cheapy aftermarket spring/weight kit and dont really know what to be using. i ditched the stock weights a while ago for whatever can in the package, and just put medium springs in i belive. ive never been able to get normal timing read outs. probably cause its a stock balancer and it most likely has moved. but when set the mech timing was about 14 and advanced read 45. which seemed weird (cause i had never done anything with it before) but that is where it seemed to run best so i left it. anyone have any idea?
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Post by 83longroof » Tue Jan 11, 2005 9:21 am

this is hard to answer, it depends on how soon you want total timing. which depends rear end ratio, how much cam, headers or not (less exhaust gas in the fresh charge) the weight of the car, and on and on. What and why one combination of springs works for me and not for someone else, depends on too many things. Best thing to do is first as you mentioned make sure your vibration dampener is correct, you can sometimes use measurements based on the keyway. If thats right, with a timing light and a tach see what RPM the timing starts to move and what RPM it is at total. A good place to start is around 2500, the carb will be in the high circuit, and don't forget to do this with the vac.advance disconnected, and the port pluged. After that if it is too soon you need to start switching springs. Once you have a baseline test and tune night at you local track, if you want can help dial it in for whats best for you. Keep notes on what the timing is at diferent RPM and as you make changes you can track them.


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Post by Phantom » Sun Dec 04, 2005 7:34 am

by Dragginwagon
Personally with your combo, and I'm "assuming" it's not regularly street driven, more of a weekend warrior, then I'd lock out the dist.

I second that.
I locked mine out this year and should have done it a long time ago.
Racing is an addiction that some people just don't understand.


Post by loonieymike » Sun Apr 23, 2006 5:11 pm

how do you set the timing above 12 degrees when the timing tab only goes to 12? i am reading in posts that it may need to be set alot higher than that


Re: I have neither

Post by 406Regal » Tue Jun 13, 2006 1:30 am

Excelerater wrote:My distrub is locked at 38 degrees


Best Put for>> Performance :D

If you Have a Hard Start Issue with Timing Locked Out It's Not becuase of the Timing Locked out,
You have a Weak Battery,Poor Connections,Weak Starter,Etc.

14to1+ 406 NO Cranking Issues at 38 Degrees Locked
280-284* Duration @ .050
106 L/S
.683-.688 Gross Lift

Don;t Matter whether It's a Tow Truck,Daily Driver,Etc.
NO engine will Have better Throttle Response thatn that of one is Locked out,
As for The Lean Idle Mixture at Idle with Locked out Timing >>
Simply Adjust Idle Mixture Screws to fatten it up at Idle.

It's like this NO Matter how it was explained in the Looooong Post
If a Engine is Running It's Timing at say 20* Timing it WILL Be Running RICHER at a Idle.... Thus seeing Idle cooling Capability.

Now say you run it at 40^ Timing Locked out,
Of course It's gonna want More fuel at a Idle as It's Lighting the Fuel Sooner,
In short Fatten Idle Mixture up by backing out Mixture screws.....
You've Now solved the Idle Cooling Capability and WILL Have Much Better Throttle response,
Fattening up Idle Mixture Plays NO Role in Fuel Economy Once Throttle Blades get opened up (Above 1800 RPM's)

Hope this clarifies things for those looking for Optimum Performance 8)

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nice article

Post by xtremeclonedss » Tue Jun 13, 2006 7:42 am

All I have to contribute to this is that for a beginner that is trying to learn and understand as much as possible it is nice to have guys explain stuff in simple language where you can understand it.

Thanks for the explanation.
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Soon I hope: 406 with a 4 speed and interior complete.


Post by kybowbender » Mon Jun 19, 2006 9:55 pm

I just purchased a new holley 650 double pumper and the instructions say to hook the vacuum advance on the ported vacuum port up above the idle mixture screw. After reading this I'm kind of confused. A reputable company like holley don't know where you are suppose to hook the vacuum advance?

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